ABSTRACT Prions (or PrPSc) are the causal agents of fatal transmissible prion diseases including sporadic Creutzfeldt-Jakob disease (sCJD, the most common human prion disease) in humans as well as scrapie, mad cow disease and chronic wasting disease in animals. sCJD is transmissible via medical or surgical procedures due to contamination by abundant infectious prions in the brain of patients. Notably, some epidemiological studies have also associated sCJD risk with non-neurosurgeries but experimental evidence for such a link is lacking. sCJD is currently incurable. At the onset of clinical symptoms, permanent brain damages already occurred. The absence of less invasive early diagnostic tests for the disease can result in missing the critical window for future treatments, and low brain autopsy rate due to cultural constraints prevents the surveillance of sCJD that is essential for effective prevention of iatrogenic sCJD transmissions. Our recent study using the highly sensitive real-time quaking-induced conversion (RT- QuIC) assay and humanized transgenic (Tg) mice-based bioassay revealed that the skin of sCJD patients harbors infectious prions (Orr et al., 2017). Our new preliminary results further indicate that skin PrPSc is detectable by both RT-QuIC and serial protein misfolding cyclic amplification (sPMCA) assays even at the asymptomatic stage in a prion-infected animal model. We hypothesize that skin PrPSc can be both a source of iatrogenic prion transmission and a biomarker for preclinical/premortem/postmortem diagnostic testing of prion diseases. To test for the hypothesis, the following four Aims will be pursued: (1) to quantitate infectivity of skin prions from sCJD patients, (2) to pinpoint the distribution of PrPSc within the skin, (3) to evaluate the potential of skin PrPSc as the source of iatrogenic transmission, and (4) to validate skin PrPSc as a biomarker for diagnosis of prion diseases. We expect that the proposed study will not only shed light on the potential risk of human-to-human sCJD transmission via skin prions but also establish alternative preclinical/premortem/postmortem diagnostic assays for prion diseases. Moreover, new knowledge generated from this study may apply to much more common neurodegenerative diseases such as Alzheimer?s and Parkinson?s diseases where the disease-specific misfolded proteins have been observed in the skin of respective patients.